The present invention relates to a compensator to compensate for the changes in length caused by temperature effects of fixed installations, such as lines, flow channels and the like through which a product flows.
In the case of many polymer processing machines which melt granules or pellets and feed them into a common slot die in order to coextrude a multilayer film, the difficulty arises that, owing to the flow of melt at temperatures of up to 300.degree. C., there occur thermal expansions of the melt lines which connect coextruders and extruders to an adapter and/or a multilayer die or slot die. In this case, when the melt zone is heated to operating temperatures in the range of up to 300.degree. C., the thermal expansion can cause the length of melt lines to change by about up to 2.8 mm/linear meter. These changes in length or thermal expansions of the melt lines take place according to the formula EQU 1.sub.w =1.sub.k +(1.sub.k .multidot..DELTA.t.multidot.c)=1.sub.k +(1.sub.k .multidot.250.degree. C..multidot.11.multidot.10.sup.-6 /.degree. C.)
This change in length, starting from a fixed point, displaces the coextruders rearward with respect to the slot die. Known installations with extruders and coextruders are therefore constructed in such a way that the coextruders can be moved in the axial direction, on condition that the melt lines likewise run in or parallel to the axial direction.
In the case of coextruders which are arranged at right angles to the film take-off from the slot die and are set up in this way to save space, these changes in length of the melt lines cause leakages to occur at the 90.degree. bends of the melt lines, and tensioning chains may be destroyed, with the correspondingly associated risk of accidents. In order to avoid such problems, the coextruders can be moved in the direction of their transverse axis to correct them according to the thermal expansion of the melt lines. However, this necessitates complex displacing devices, combined with corresponding distance measuring systems. Considerable extra costs per extruder installation are thereby engendered, since it is necessary to compensate for linear expansions in the range of 15 to 25 mm per coextruder.
For use in a completely different context, namely for deflecting a melt flow, so-called start-up valves are known. These are used in a start-up position for deflecting the melt flow out of the extruder into the open during the starting phase, in order to discharge unwanted material. After this purging operation, the valve is switched back into its operating position. The known start-up valve has a hydraulic system which makes it possible to switch the valve over into the respectively required operating position while the extruder is running.